Drive current supply circuit with current mirror

ABSTRACT

A drive current supply circuit is provided in which the speed of rise of the drive current to a laser diode is increased. In a drive current supply circuit for supplying drive current I to load  50  connected with the line  1 R of two parallel lines  1 R and  2 R of a first current mirror circuit  1 , this drive current supply circuit is provided with a control circuit CONT that controls the current i flowing in the other line  1 L in accordance with first control signal Σø 1  and the first control signal Σø 1  comprises a steady DC component øD and a drive signal component ø 1  added to the DC component øD when desired.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a drive current supply circuit.

2. Description of the Related Art

Drive current supply circuits have been developed for laser diodes inoptical pickups. Prior art drive current supply circuits are disclosedin published Japanese Patent No. H. 7-31823 and U.S. Pat. No. 5,898,334.The drive current to the laser diode in an optical pickup is differentduring writing to the storage medium and during reading, being set to alarger value during writing. The drive current supply circuits describedabove are effective when the drive current is to be made variable.

SUMMARY OF THE INVENTION

However, in the conventional drive current supply circuits, when thedrive current of the laser diode is increased when writing to thestorage medium, the speed of its rise is low, leading to the problemthat, when reading and writing are repeated, high-speed writing cannotbe performed. In light of the foregoing, an object of the presentinvention is to provide a drive current supply circuit wherein the speedof rise of the drive current to the laser diode constituting the loadcan be increased.

In order to solve the problem described above, a drive current supplycircuit according to the present invention that supplies drive currentto a load connected with one of the lines of a first current mirrorcircuit having two parallel lines comprises a control circuit thatcontrols the current flowing in the other of the lines in accordancewith a first control signal (the potential of the other line) so thatthe first control signal consists of a steady DC component and a drivesignal component added to this DC component when desired.

That is, the DC component is applied to the line when reading data froman optical disk such as CD-R, CD-RW, DVD-R or DVD-RW, and the drivesignal component is applied to the line when writing data on the opticaldisk. The drive signal component may be a signal modulated by usingeight to fourteen modulation (EFM). The DC and drive signal componentmay be modulated by high frequency.

It is known that a current mirror circuit comprises two transistors andhas two parallel lines. The current flowing through one line is equal toor proportional to the current flowing through the other line.Consequently, if the current flowing through the other line iscontrolled, the current flowing through the load connected to the oneline can be controlled. This control is performed in accordance with afirst control signal.

A transistor has a threshold voltage at which current starts to flow.When voltage of above this threshold is applied between the controlterminals of the transistor i.e. between the base/emitter in the case ofa bipolar transistor or between the source/gate in the case of afield-effect transistor, current flows in the transistor and currenttherefore flows in the load.

In a conventional control circuit that outputs such a control signal,this first control signal consists solely of a DC component duringreading and consists solely of an AC component during writing.Consequently, in the control circuit, the conventional AC signal duringwriting constitutes the drive signal and when the current flowingthrough the other line is controlled in accordance with the magnitude ofthis, the voltage between the control terminals starts to rise from 0 Vand so can only reach the desired voltage level after passing throughthis threshold voltage.

In contrast, in the drive current supply circuit of the presentinvention, since the first control signal comprises a steady DCcomponent and a drive signal component added to this DC component whendesired i.e. during writing, the voltage between the control terminalshas already reached a certain level, due to this DC component, so theamount of variation for this to reach the drive signal component issmaller, so the rise time is shorter.

For the sake of this DC component, the level to which the voltagebetween the control terminals is to be raised can be determined asrequired. When this drive current supply circuit is employed in readingand writing by an optical pickup, during reading, it sets the DCcomponent at or above the threshold voltage of the transistor and duringwriting a drive signal component increment is superimposed on this, so avoltage of at or above the threshold voltage of the transistor is ofcourse set between the control terminals and, when writing, a largedrive current flows to the load i.e. the laser diode through one of theparallel lines in the current mirror circuit.

The word “steady” means a continuous period in which there is no changewhether in the reading period or in the writing period; it does not meanthat the DC component does not change when the power source of the drivecurrent supply circuit itself is OFF or when other control is beingperformed.

As described above, when the load is a laser diode, this drive currentsupply circuit can be utilized in an optical pickup.

The current mirror circuits could also be constructed of bipolartransistors, but, from the point of view of reducing power consumption,preferably consist of field-effect transistors.

In this case, the first current mirror circuit may comprise twofield-effect transistors with their gates connected in common, onechannel of the field-effect transistors being the one of the lines, theother channel of the field-effect transistors being the other of thelines, the DC voltage applied to the gate being the DC component of thefirst control signal and the drive voltage applied to the gate being thedrive signal component of the first control signal.

When the gate/source voltage in the field-effect transistors iscontrolled i.e. when DC voltage or drive voltage is applied by thecontrol circuit, the current flowing in the other line of the currentmirror circuit changes depending on this first control signal. Since, asdescribed above, the first control signal is set such that the riseduring writing is speeded up, the rise of the drive current supplied tothe load is speeded up.

Also, by preparing a plurality of current mirrors of the sameconstruction as ascribed above and forming a node by connecting all ofthe one sides of the respective parallel lines, the load being connectedwith this node, the currents flowing through the other side of theparallel lines of each of the current mirror circuits are added.Consequently, the current flowing to the load can be controlled as thesum of these by individually controlling by means of the control circuitthe currents that flow through the other sides of the parallel lines ofthe current mirror circuits.

Specifically, the drive current supply circuit in this case may furthercomprise at least a second current mirror circuit having two parallellines, the one of the lines of the second current mirror circuit beingconnected with the load and wherein the control circuit controls thecurrent flowing through the other of the lines of the second currentmirror circuit in accordance with a second control signal, the secondcontrol signal comprising a steady DC component and, when desired, adrive signal component added to this DC component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a circuit diagram of a drive current supply circuit accordingto an embodiment;

FIG. 1B is a circuit diagram of one of control circuits 11 to 41 shownin FIG. 1A.

FIG. 1C is a circuit diagram of optical disk player.

FIG. 2 is a circuit diagram of a drive current supply circuit whereinthe conventional circuit is somewhat improved;

FIG. 3 is a graph showing the relationship of gate/source voltage (gatevoltage) and drain current in a field-effect transistor;

FIG. 4 is a timing chart in the circuit of FIG. 2; and

FIG. 5 is a timing chart in the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A drive current supply circuit according to an embodiment is describedbelow. Identical elements are given the same reference symbols to avoidduplicated description.

FIG. 1A is a circuit diagram of a drive current supply circuit accordingto an embodiment and FIG. 1B is a circuit diagram of one of controlcircuits 11 to 41 shown in FIG. 1A. In a drive current supply circuitwherein a drive current is applied to a load 50 connected with one side(1R) of the lines of the first current mirror circuit 1 provided withtwo parallel lines 1R, 1L, this drive current supply circuit comprises acontrol circuit CONT that controls the current i flowing in another-side one (1L) of the lines, in accordance with a first controlsignal Σø1. The load 50 is a laser diode.

This first control signal Σø1 consists of a steady DC component øD and adrive signal component ø1 obtained by adding the increment Δø1 of thedrive signal component to the DC component øD when desired. It should benoted that “adding” is not restricted to addition of physical quantitieswhich are both of positive sign.

This first control signal Σø1 is the potential of the gate of currentmirror circuit 1. The DC component øD is applied to the line viaresistance R1 when reading data from an optical disk such as CD-R,CD-RW, DVD-R or DVD-RW, and the drive signal component ø1 is applied tothe line during writing data to the optical disk. The drive signalcomponent may be a signal modulated by using eight to fourteenmodulation (EFM). The DC signal component and drive signal component maybe modulated by high frequency.

DVD requires 650 nm wavelength laser light for reading data, and CDrequires 780 nm wavelength laser light for reading data. So, if the load50, laser diode is applied to DVD/CD compatible player, the laser diodeis comprised of two laser diodes, the respective diodes emitting 650 nmand 780 nm wavelength laser light, respectively.

Current mirror circuit 1 comprises two transistors 1QR, 1QL and isprovided with two parallel lines 1R, 1L. The current flowing throughone-side line 1R is equal to or proportional to the current flowingthrough the other-side line 1L. Consequently, by controlling the currentflowing through the other-side line 1L, the current flowing through theload 50 connected with the one-side line 1R can be controlled. Thiscontrol is performed in accordance with the first control signal Σø1.

The current i₁ flowing in line 1L of first current mirror circuit 1 isdetermined by the DC component øD supplied from common partial controlcircuit 11 within control circuit CONT and the drive signal component ø1supplied from first partial circuit 21. When the first control signalΣø1 becomes drive signal component ø1 as a result of addition of drivesignal component increment Δø1 (negative) to the DC component øD, thefirst control signal Σø1 becomes small, but since, in this example, weare considering the potential difference from this source to the gate ofa transistor of high potential, the voltage obtained by subtractingfirst control signal Σø1 from the source potential becomes large, socurrent i₁ is increased. Likewise, the currents i₂ and i₃ flowing in thelines 2L and 3L of the second and third current mirror circuits 2, 3 aredetermined by the DC component øD supplied from the common partialcontrol circuit 11 within control circuit CONT and the drive signalcomponents ø2, ø3 supplied from the second and third partial circuits31, 41. The operation is the same as in the case of the first currentmirror circuit 1.

In other words, the drive current supply circuit of this example isfurther provided with a second current mirror circuit 2 having at leasttwo parallel lines, the one-side line 2R of the lines of this secondcurrent mirror circuit 2 being connected with a load 50; control circuitCONT controls the current flowing through the other-side line (2L) ofthe lines of second current mirror circuit 2 in accordance with a secondcontrol signal Σø2; this second control signal Σø2 consists of a steadyDC component øD and a drive signal component ø2 that is added to this DCcomponent when desired.

Likewise, the one-side line 3R of the lines of a third current mirrorcircuit 3 is connected with a load 50; control circuit CONT controls thecurrent i3 flowing through the other-side line (3L) of the lines ofthird current mirror circuit 3 in accordance with a third control signalΣø3; this third control signal Σø3 consists of a steady DC component øDand a drive signal component ø3 that is added to this DC component whendesired.

Common partial control circuit 11, first partial control circuit 21,second partial control circuit 31 and third partial control circuit 41are function conversion circuits respectively comprising input terminals(12, 22, 32, 42), control input terminals (13, 23, 33, 43) and outputterminals (terminals that supply øD, ø1, ø2 and ø3). The outputs supplyøD, ø1, ø2 and, ø3 are expressed by functions of the input to inputterminals 12, 22, 32, 43, suitably by a proportionality relationshipfunction. Control inputs 13, 23, 33, 43 input signals that controlON/OFF of the outputs øD, ø1, ø2 and ø3.

In the circuit shown in FIG. 1A, there are provided a plurality ofcurrent mirror circuits 2, 3 of the same construction as current mirrorcircuit 1.

Current mirror circuit 1 is provided with transistors 1QR, 1QL and lines1R, 1L and is supplied with DC component øD and drive signal componentø1 and current i₁ flows in the lines 1L and 1R. In current mirrorcircuit 2, replacing the 1 of the symbols used in current mirror circuit1 by 2, there are provided transistors 2QR, 2QL and lines 2R, 2L and itis supplied with DC component øD and drive signal component ø2 andcurrent i₂ flows in the lines 2L and 2R.

In current mirror circuit 3, replacing the 1 of the symbols used incurrent mirror circuit 1 by 3, there are provided transistors 3QR, 3QLand lines 3R, 3L and it is supplied with DC component øD and drivesignal component ø3 and current i₃ flows in the lines 3L and 3R.

Since the constructions of the second and third current mirror circuits2 and 3 are the same as the construction of first current mirror circuit1, hereinbelow, the first current mirror circuit 1 will be described indetail.

Transistors 1QR and 1QL in first current mirror circuit 1 are p-channelfield-effect transistors, their sources being connected with +V voltpower source potentials 25, 26 and their gates being connected incommon. The gate and drain of transistor 1QL are short-circuited so thatthe gate voltage is controlled by the potential applied to the drain. Inthe case of p-channel field-effect transistors, the transistor is in ONcondition when the voltage between the gate and source is set such thatthe gate potential is relatively negative with respect to the source.

Consequently, as described above, the current i₁ (drive current I) iscontrolled in accordance with the first control signal Σø1 but, in thisexample, more precisely, the current i₁ is controlled in accordance withthe voltage obtained by subtracting the gate (drain) potential Σø1 fromthe source potential (+V) in transistor 1QL.

Transistors 1QR and 1QL have a threshold voltage at which current startsto flow. This threshold value is set by the voltage between the gate andsource in transistor 1QL i.e. by the drain/source voltage.

When greater voltage than or equal to the threshold voltage oftransistor 1QL (|V−Σø1|) is applied between the control terminals of thetransistor i.e. when it is applied between the gate and source in thefield-effect transistor, a current i₁ flows in transistor 1QL and thesame current I also flows in transistor 1QR which shares this gate, sodrive current flows in load 50.

Current mirror circuit 1 may be constituted of bipolar transistors but,with a view to reducing power consumption, is preferably constituted offield-effect transistors. However, if these transistors are bipolartransistors, the source, gate and drain referred to in the context offield-effect transistors may be respectively substituted by emitter,base and collector and in this case the voltage between theaforementioned control terminals therefore becomes the voltage betweenbase and emitter.

The load 50 in this example is a laser diode; this is employed in anoptical pickup for reading from a storage medium and writing.

FIG. 1C is a circuit diagram of an optical disk player. The input datafor writing is encoded by an encoder PL1 such as EFM circuit. Theencoded data is applied to for example the control terminal 23 of thedrive current supply circuit PL2 shown in FIG. 1A while applying DCvoltage to the terminal 13. The load 50, laser diode emits laser lightin response to the encoded data.

When the optical disk OPD is illuminated by the laser light beam forwriting, a recording layer of the optical disk OPD changes its physicalstructure and the data is recorded in the optical disk OPD. Thereflected laser light beam by the optical disk OPD is detected by aphotodiode PL3.

When the optical disk OPD is illuminated by the laser light beam forreading that is modulated by the high frequency, the electric outputfrom the photodiode PL3 is inputted into a data regenerative circuit PL4and the recorded data is extracted. The electric output is also appliedto an auto power control (APC) circuit PL5 that controls the powerapplied to the laser diode 50. The APC circuit PL5 is controlled by acontrol circuit PL6. Note that such an APC control is used duringwriting, too.

The control circuit PL6 also controls a spindle motor SM for rotatingthe optical disk OPD and a linear motor LM for moving the opticalpick-up unit OPU including laser diode 50.

When the photodiode PL3 has multi-divided photosensitive areas, theelectrical output from the photodiode PL3 may be used for focusing thelaser light beam. This focusing technique is conventional.

The control circuit PL6 decides the laser power by the required physicalstatus of the recording layer when the optical disk is CD-RW. Forexample, a middle power heating by laser light beam and gradual coolingcauses the recording layer to become a crystalline. High power heatingand rapid cooling causes the recording layer to become amorphous.

The control circuit PL6 may decide the laser power by the kind of theoptical disk OPD because the required laser power differs by the kind ofthe optical disk.

In this drive current supply circuit shown in FIG. 1A, since the firstcontrol signal Σø1 consists of a steady DC component øD and a drivesignal component ø1 added to this DC component øD when desired i.e. whenwriting, from the standpoint of the source, the voltage across thecontrol terminals has already reached a certain negative level becauseof this DC component øD, so, when drive signal component ø1 is generatedby addition of the drive signal component increment Δø1 thereto, itsrise time in the negative direction is shortened. In terms of potential,since the potential of the DC component øD is lower than +V, thepotential of the drive current component ø1 is lower than the DCcomponent øD.

For example, if +V is set at 5 V, the DC component øD is chosen to be 4V, and the center of amplitude of the drive signal component ø1constituting the pulse signal is set at 2.5 V. In the case of the drivesignal component ø1 only, in order to cause a current i₁′ as shown inFIG. 2, to be described, to pass, ø1 must be lowered from 5 V to 2.5 V;thus, regarding the difference of these as the gate/source voltage, fromthe standpoint of the source, a voltage of −2.5 V must be applied to thegate.

If, as in the case of the circuit shown in FIG. 1A, a potential of forexample 4 V is applied beforehand to the gate as øD, from the standpointof the source, a voltage or −2.5 V can be applied to the gate simply bylowering ø1 from 4 V to 2.5 V. Since stray capacitance is of coursepresent between the gate and source of the transistor, when the voltagebetween the gate and source is changed, some time is required forcharging/discharging of this capacitance. Therefore, an excellentresponse characteristic of the output current can be obtained by makingthe change of the voltage between the gate and source small.

Although, in the drawing, a resistance R1 is inserted on a lineconnecting the line that supplies ø1 and the line that supplies øD, thisresistance could be inserted in partial control circuit 11.

The level to which the voltage between the control terminals should beraised is determined as needed by DC component øD. This drive currentsupply circuit, when employed for reading and writing by an opticalpickup, in the case of reading, sets DC component øD of at or above thethreshold voltage of transistor 1QL so that normally drive current Iflows; in the case of writing, it adds an increment thereto, replacingit with drive signal component ø1; thus a voltage of at or above thethreshold voltage of transistor 1QL is of course set between the controlterminals, so that, in the case of writing, a large current flows toload 50 i.e. the laser diode through one side of the parallel lines inthe current mirror circuit 1.

It should be noted that this threshold voltage is set lower than thevoltage that applies a threshold value of drive current I that producesoscillation of laser diode (50).

The word “steady” means a continuous period in which there is no changewhether in the reading period or in the writing period; it does not meanthat the DC component does not change when the power source of the drivecurrent supply circuit itself is OFF or when other control is beingperformed.

As described above, this drive current supply circuit can be utilized inan optical pickup when the load 50 is a laser diode.

Referring to the first current mirror circuit 1, first current mirrorcircuit 1 comprises two field-effect transistors 1QR, 1QL with theirgates connected in common, the one-side channel of the field-effecttransistors being designated as the one-side line (1R), the other-sidechannel of the field-effect transistors being designated as theother-side line (1L), the DC voltage øD that is applied to the gatebeing designated as the DC component of the first control signal, andthe drive voltage ø1 that is applied to the gate being designated as thedrive signal of the first control signal. Control circuit CONT appliesthe DC voltage and the drive voltage to the gate.

When a voltage is applied between the gate and source in field-effecttransistor 1QL i.e. when DC voltage øD and drive voltage ø1 are appliedby control circuit CONT, since the current i₁ flowing through theother-side line 1L of current mirror circuit 1 changes depending on thisfirst control signal Σø1, the first control signal Σø1 being set, asdescribed above, so that the rise in the case of writing is speeded up,the rise of the drive current I supplied to load 50 is speeded up.

It should be noted that the DC component øD is supplied from commonpartial control circuit 11 to each of the lines 1L, 2L, 3L and, ifrequired, resistances R1, R2, R3 may be inserted as shown in the Figureon each line.

A node 51 is formed by connection of all of 1R, 2R, 3R on one side ofthe parallel lines of the respective current mirror circuits 1, 2, 3.Load 50 is connected to node 51, so the currents i₁, i₂, i₃ flowingthrough the other sides 1L, 2L, 3L of the parallel lines of each of thecurrent mirror circuits 1, 2, 3 are added i.e. the currents i₁, i₂, i₃flowing through the one-side lines 1R, 2R, 3R are added and supplied toload 50 as drive current I.

Since the currents flowing through the other sides 1L, 2L, 3L of theparallel lines of each of the current mirror circuits 1, 2, 3 areindividually controlled by control circuit CONT, the current I flowingto load 50 can be controlled as the sum of these.

It should be noted that, in the prior art control circuit, the controlsignals supplied to the specified lines in a single current mirrorcircuit consisted solely of a DC component in the case of reading andconsisted solely of an AC component modulated by the write informationin the case of writing.

FIG. 2 is a circuit diagram of a case in which this prior art circuit issomewhat improved. For the structural elements in FIG. 2 the samesymbols are employed as in the case of FIG. 1A, so detailed descriptionthereof is omitted, but their connection relationship differs as shown.This circuit employs a plurality of current mirror circuits 1, 2, 3, D;current mirror circuit D that is supplied solely with DC component øDwhen reading is performed and current mirror circuits 1, 2, 3 that aresupplied solely with AC components modulated by the write informationwhen writing is performed are independently provided; drive current I toload 50 is constituted by adding the output currents i₁′, i₂′, i₃′,i_(D)′ these current mirror circuits 1, 2, 3, D.

In a circuit as shown in FIG. 2 also, when control circuit CONT′controls the current i₁′ flowing through the parallel lines of aspecified current mirror circuit 1 in accordance with drive signalcomponent ø1 consisting solely of an AC signal during performance ofwriting, directing attention to the specified current mirror circuit 1that is supplied with this drive signal component, from the point ofview of the source, the voltage between the control terminals (gate andsource) starts to rise in the negative direction from 0 V and, exceedingthis threshold voltage, attains a desired voltage level. Consequently,in comparison with the embodiment described above, the speed of rise ina circuit constructed as above is slow.

The principle whereby the rise as described above is speeded up will nowbe described in detail.

FIG. 3 is a graph showing the relationship between the gate/sourcevoltage (gate voltage) and the drain current in a field-effecttransistor.

The drain currents I_(A), I_(B), I_(C) (=I_(A)+I_(B)) increasemonotonically in proportion to the gate voltages V1, V2, V3. As in thecase of the circuit shown in FIG. 2, when individual gate voltages V1,V2 are applied to the transistors in the individual current mirrorcircuits D, 1, when writing, drive current I=I_(C)=I_(A) (i_(D)′)+I_(B)(i₁′) can be obtained as the sum of the drain currents, but, in order toobtain current I_(B) (i₁′), the gate voltage must be increased from 0 Vto V2.

In contrast, in a drive current supply circuit according to theembodiment described above, since gate voltage V1 (V−øD) is alreadyapplied to the gate of transistor 1QL in a single current mirror circuit1, when writing, in order to obtain a current I_(C) (i₁) by adding V3(V−Σø1 (=V−ø1)), it suffices merely to raise the gate voltage front V1to V3, so a speeding up can be achieved.

FIG. 4 is a timing chart in the circuit of FIG. 2.

In the circuit shown in FIG. 2, during reading etc, DC current øD isapplied steadily from time-point T1 (input to control input terminal13=H) and in a desired period such as during writing (time-point T2 toT3), drive signal component ø1 is applied (input to control inputterminal 23=H). In this case, after time-point T2, the period Td beforethe drive current starts to rise, the period Tr in which it has risen,and the period Tf of falling after time-point T3 are comparatively long.

FIG. 5 is a timing chart in the circuit of FIG. 1A.

In contrast, in the circuit shown in FIG. 1A, during reading etc, DCcurrent øD is applied steadily from time-point T1 (input to controlinput terminal 13=H) and in a desired period such as during writing(time-point T2 to T3), drive signal component ø1 is applied (input tocontrol input terminal 23=H). But in this case, after time-point T2, theperiod Td before the drive current starts to rise, the period Tr inwhich it has risen, and the period Tf of falling after time-point T3 areall comparatively shorter than the periods shown in FIG. 4, sohigh-speed rising and falling can be achieved.

Thus, when an optical pickup is employed that uses the circuit describedabove in a writable optical disc, the timing of writing can be such thatthis is performed continuously with the condition in which the disc wasbeing read, when the optical disc is rotating, a condition is therebyproduced in which the current for generating the laser beam for readingis always flowing. A condition in which control input terminal 13 isplaced beforehand in ON condition so that a steady DC drive current Iflows is often employed in actual optical pickups. With the use ofhigher speeds of writing in writing information to disks, the responsecharacteristic of the laser beam is important; if the drive currentsupply circuit of the embodiment described above is employed, rise ofthe laser diode constituting load 50 can be achieved rapidly, so, in anoptical pickup in which reading and writing are performed frequently,the writing speed can be increased.

Although, in the above, three current mirror circuits were employed,these could be four or more and although the output current of thecurrent mirror circuits in each stage were equally divided, they couldbe different.

As described above, with the drive current supply device of the presentinvention, the speed of rise of the drive current to the laser diodeconstituting the load can be increased.

1. A drive current supply circuit for supplying drive current to a laserdiode used for reading data from and writing data on an optical disk,comprising: a first current mirror circuit having two parallel lines,said laser diode being connected with one of the two parallel lines; anda control circuit connected with the other of the two parallel lines,said control circuit controlling the current flowing in this line inaccordance with a potential of this line, this potential comprising asteady DC component when reading data; and this potential comprising adrive signal component added to said DC component when writing data,wherein an amount of variation of the potential to reach the drivesignal component is smaller, and the rise time is shorter, than avariation of the potential that consists solely of an AC component. 2.The drive current supply circuit according to claim 1, wherein saidfirst current mirror circuit comprises first and second field-effecttransistors with their gates connected in common, wherein the channel ofsaid first field-effect transistor is said one of said lines, andwherein the channel of said second field-effect transistor is said otherof said lines.
 3. The drive current supply circuit according to claim 1,further comprising a second current mirror circuit having two parallellines, one of said lines of said second current mirror circuit beingconnected with said laser diode, wherein said control circuit controlsthe current flowing through the other of said lines of said secondcurrent mirror circuit in accordance with a potential of the other ofsaid lines, this potential comprising a steady DC component when readingdata; and this potential comprising a drive signal component added tosaid DC component when writing data.